Field of the Invention
This invention relates to wireless communication systems, and to a method and apparatus of bandwidth request/grant protocols in a broadband wireless communication system.
Description of Related Art
As described in the commonly assigned U.S. Pat. No. 6,016,311, issued Jan. 18, 2000, titled “An Adaptive Time Division Duplexing Method and Apparatus for Dynamic Bandwidth Allocation within a Wireless Communication System,” which is hereby incorporated by reference, a wireless communication system facilitates two-way communication between a plurality of subscriber radio stations or subscriber units (fixed and portable) and a fixed network infrastructure. Exemplary communication systems include mobile cellular telephone systems, personal communication systems (PCS), and cordless telephones. The key objective of these wireless communication systems is to provide communication channels on demand between the plurality of subscriber units and their respective base stations in order to connect a subscriber unit user with the fixed network infrastructure (usually a wire-line system). In the wireless systems having multiple access schemes a time “frame” is used as the basic information transmission unit. Each frame is sub-divided into a plurality of time slots. Some time slots are used for control purposes and some for information transfer. Subscriber units typically communicate with the base station using a “duplexing” scheme thus allowing the exchange of information in both directions of connection.
Transmissions from the base station to the subscriber unit are commonly referred to as “downlink” transmissions. Transmissions from the subscriber unit to the base station are commonly referred to as “uplink” transmissions. Depending upon the design criteria of a given system, the prior art wireless communication systems have typically used either time division duplexing (TDD) or frequency division duplexing (FDD) methods to facilitate the exchange of information between the base station and the subscriber units. Both the TDD and FDD duplexing schemes are well known in the art.
Recently, wideband or “broadband” wireless communications networks have been proposed for providing delivery of enhanced broadband services such as voice, data and video services. The broadband wireless communication system facilitates two-way communication between a plurality of base stations and a plurality of fixed subscriber stations or Customer Premises Equipment (CPE). One exemplary broadband wireless communication system is described in the incorporated patent, U.S. Pat. No. 6,016,311, and is shown in the block diagram of FIG. 1. As shown in FIG. 1, the exemplary broadband wireless communication system 100 includes a plurality of cells 102. Each cell 102 contains an associated cell site 104 that primarily includes a base station 106 and an active antenna array 108. Each cell 102 provides wireless connectivity between the cell's base station 106 and a plurality of customer premises equipment (CPE) 110 positioned at fixed customer sites 112 throughout the coverage area of the cell 102. The users of the system 100 may include both residential and business customers. Consequently, the users of the system have different and varying usage and bandwidth requirement needs. Each cell may service several hundred or more residential and business CPEs.
The broadband wireless communication system 100 of FIG. 1 provides true “bandwidth-on-demand” to the plurality of CPEs 110. CPEs 110 request bandwidth allocations from their respective base stations 106 based upon the type and quality of services requested by the customers served by the CPEs. Different broadband services have different bandwidth and latency requirements. The type and quality of services available to the customers are variable and selectable. The amount of bandwidth dedicated to a given service is determined by the information rate and the quality of service required by that service (and also taking into account bandwidth availability and other system parameters). For example, T1-type continuous data services typically require a great deal of bandwidth having well-controlled delivery latency. Until terminated, these services require constant bandwidth allocation on each frame. In contrast, certain types of data services such as Internet protocol data services (TCP/IP) are bursty, often idle (which at any one instant requires zero bandwidth), and are relatively insensitive to delay variations when active.
Due to the wide variety of CPE service requirements, and due to the large number of CPEs serviced by any one base station, the bandwidth allocation process in a broadband wireless communication system such as that shown in FIG. 1 can become burdensome and complex. This is especially true with regard to the allocation of uplink bandwidth. Base stations do not have a priori information regarding the bandwidth or quality of services that a selected CPE will require at any given time. Consequently, requests for changes to the uplink bandwidth allocation are necessarily frequent and varying. Due to this volatility in the uplink bandwidth requirements, the many CPEs serviced by a selected base station will need to frequently initiate bandwidth allocation requests. If uncontrolled, the bandwidth allocation requests will detrimentally affect system performance. If left unchecked, the bandwidth required to accommodate CPE bandwidth allocation requests will become disproportionately high in comparison with the bandwidth allocated for the transmission of substantive traffic data. Thus, the communication system bandwidth available to provide broadband services will be disadvantageously reduced.
Some prior art systems have attempted to solve bandwidth allocation requirements in a system having a shared system resource by maintaining logical queues associated with the various data sources requiring access to the shared system resource. Such a prior art system is taught by Karol et al., in U.S. Pat. No. 5,675,573, that issued on Oct. 7, 1997. More specifically, Karol et al. teach a bandwidth allocation system that allows packets or cells within traffic flows from different sources that are contending for access to a shared processing fabric to get access to that fabric in an order that is determined primarily on individual guaranteed bandwidth requirements associated with each traffic flow. In addition, the system taught by Karol et al. allow the different sources to gain access to the shared processing fabric in an order determined secondarily on overall system criteria, such as a time of arrival, or due date of packets or cells within the traffic flows. Packets or cells of data from each data source (such as a bandwidth requesting device) are queued in separate logical buffers while they await access to the processing fabric.
The bandwidth allocation techniques described in the commonly assigned and incorporated U.S. patent application Ser. No. 09/316,518, filed May 21, 2000, utilizes mechanisms referred to as “bandwidth request/grant protocols” to provide on-demand bandwidth needs of individual CPE connections. Typically, bandwidth request/grant protocols operate in accordance with the following description. A CPE typically transmits a bandwidth request to an associated base station. The request identifies the aggregate (i.e., the total) bandwidth needs of the connection. The base station receives the bandwidth request and determines whether sufficient bandwidth is available to grant the bandwidth request. If sufficient bandwidth is available, the requested bandwidth is granted to the connection, else the base station waits for sufficient bandwidth to become available before granting the requested bandwidth. As described in the parent patent application, bandwidth request/grant protocols improve bandwidth allocation efficiencies in wireless communication systems under ideal conditions.
However, as is well known, bandwidth requests (and associated grants) can be lost (i.e., never received by the associated base station) or delayed due to noise and interference effects inherent to all wireless communication systems. When bandwidth requests are lost or delayed during transmission between a CPE and a base station, bandwidth allocation efficiencies can be adversely affected. Lost or delayed bandwidth requests contribute to the reduction of bandwidth allocation efficiency in wireless communication systems by causing the base stations to inaccurately allocate bandwidth to their associated and respective CPEs.
For example, consider the situation where a selected CPE transmits a bandwidth request to its associated base station wherein the request identifies the aggregate bandwidth requirements of the selected CPE. Assume that the bandwidth request is lost in transmission due to interference on the air link between the base station and the selected CPE. In this example, the associated base station never receives the aggregate bandwidth requirements of the selected CPE, and the base station therefore never grants the CPE's bandwidth request. After waiting a suitable period of time, the CPE will determine that is has not received a bandwidth grant from the base station. Disadvantageously, the CPE will be unable to determine if the bandwidth request was lost during transmission or if the base station simply did not have sufficient bandwidth to grant the request (given the quality of service (“QoS”) of the associated connection).
The CPE may then transmit a second bandwidth request for the same connection. Under certain conditions, a “race condition” may occur that could cause the bandwidth allocation technique to waste the allocation of bandwidth. If the timing of the bandwidth requests (and subsequent grants) is such that the selected CPE issues the second bandwidth request for the same connection concurrently with the base station's grant of the first request, the second request and the grant to the first request may be concurrently transmitted over the link. That is, if the base station transmits a grant to the first request before receiving the second request from the CPE, the base station may respond to the second request and consequently grant a duplicate bandwidth request for the same connection. This disadvantageously results in an efficient allocation of bandwidth.
One alternative bandwidth request/grant protocol that prevents the occurrence of the above-described “race condition” is the so-called “guaranteed delivery protocol.” As is well known, guaranteed delivery protocols make use of acknowledgment messages that are transmitted in response to bandwidth requests. In accordance with the guaranteed delivery protocol approach, a CPE transmits to its associated base station a bandwidth request that identifies the aggregate bandwidth needs of a selected connection. The base station receives the bandwidth request and transmits an acknowledgment to the CPE thereby communicating receipt of the bandwidth request. If an acknowledgment is not received by the CPE, the CPE retransmits the bandwidth request. Advantageously, guaranteed delivery protocols vastly reduce the possibility of the base station erroneously allocating duplicate bandwidth to the CPE (as described above), and thus, improves bandwidth allocation efficiencies. However, guaranteed delivery protocols disadvantageously require additional bandwidth necessary for transmitting acknowledgement messages between the base stations and the CPEs. Furthermore, response time associated with the allocation of bandwidth is reduced because the CPEs must wait to receive acknowledgements from their associated base stations.
Some bandwidth request/grant protocols known as “incremental bandwidth request/grant protocols” attempt to solve the above-described problems relating to data transmission efficiency by utilizing incremental bandwidth requests instead of aggregate bandwidth requests. Incremental bandwidth requests identify the additional bandwidth needs of a CPE connection. For example, in accordance with incremental bandwidth request methods, a base station may allocate 1000 units of bandwidth to an associated CPE connection. At a later time, the CPE connection may require 1,500 units of aggregate bandwidth (i.e., it may require an additional 500 units of bandwidth). In accordance with the incremental bandwidth request/grant protocol, the CPE will transmit an incremental bandwidth request to its associated base station indicating that it requires an additional 500 units of bandwidth. Upon receiving the incremental bandwidth request, the base station calculates the CPE connection's current aggregate bandwidth needs as 1500 units (1000 previously granted units+500 requested units).
Advantageously, systems using the incremental bandwidth request/grant protocols respond faster and require less bandwidth than do those using the guaranteed delivery protocols because acknowledgment messages are not required by the incremental bandwidth request/grant protocols. Disadvantageously, when an incremental bandwidth request is lost, the base station loses synchronization with the CPE connection, and thereby loses track of the aggregate bandwidth needs of the CPE. Synchronization is lost because the base stations typically calculate aggregate bandwidth needs by adding each incremental bandwidth request to the previous aggregate bandwidth needs estimate. Thus, the base station and the CPE connection will remain out of synchronization until the CPE connection is reset.
Some bandwidth request/grant protocol systems have attempted to solve bandwidth allocation requirements in a system having a shared system resource by utilizing “zero bandwidth request” (ZBR) messages. One such exemplary bandwidth allocation system is known as a zero bandwidth request message protocol system and is now described. ZBR message protocol systems utilize “padding packets” and the well-known TDMA multiplexing scheme. In the well-known TDMA multiplexing scheme, a BS designates a portion of its uplink sub-frame (i.e., bandwidth) to an associated CPE. The associated CPE transmits data to the BS on the uplink. When a CPE does not have enough uplink data to utilize its entire portion of bandwidth (i.e., it has too much bandwidth allocation), it transmits padding packets to “pad” or fill its unused portion of bandwidth. The CPE then transmits a ZBR message to its associated base station (BS) to request a reduction in the CPE's bandwidth allocation. The CPE's associated BS then reduces the CPE's bandwidth allocation accordingly.
Disadvantageously in ZBR message protocol systems, utilization of ZBR messages decreases a communication systems overall speed. Base stations and CPEs require increased processing time to process and transmit ZBR messages, respectively. A BS requires increased processing time to process ZBR messages. This disadvantage is magnified in typical communication systems because a BS typically receives ZBR messages from hundreds of associated CPEs. Thus, each BS in the communication system requires relatively large amounts of time to process these ZBR messages.
Another disadvantage of zero bandwidth request message protocol systems is that CPEs can become “confused” when deciding whether to transmit zero bandwidth requests to their associated base stations. For example, a CPE has the following status: a CG connection rate of one cell per second and a DAMA connection with no data available. When the CPE's associated BS allocates one cell within a one-second time interval the cell may not yet be available within the CPE's ATM controller queues. In accordance with the ZBR protocol system, the CPE should transmit a ZBR message because of the “no data available” status of the DAMA connection. However, the CPE does not know whether or not a CG cell is going to be sent and thus it does not know whether or not to transmit a ZBR message to its associated BS. Thus, the CPE becomes confused and can erroneously transmit or refrain from transmitting a ZBR message.
A need exists for a bandwidth request/grant protocol method and apparatus that efficiently processes and responds to bandwidth allocation requests. The bandwidth allocation method and apparatus should accommodate an arbitrarily large number of CPEs generating frequent and varying bandwidth allocation requests on the uplink of a wireless communication system. For example, in the system shown in FIG. 1, as many as one hundred CPEs may be allowed to be simultaneously active, coordinating their transmissions on the uplink. Furthermore, the system can accommodate approximately one thousand CPEs on the physical channel. Such a bandwidth allocation method and apparatus should be efficient in terms of the amount of bandwidth consumed by the bandwidth request and grant messages that are exchanged between the plurality of base stations and the plurality of CPEs. That is, the plurality of bandwidth requests generated by the CPE should consume a minimum percentage of available uplink bandwidth. In addition, the bandwidth allocation method and apparatus should respond to the bandwidth allocation requests in a timely and accurate manner. The method and apparatus should be responsive to the needs of a particular communication link. The bandwidth needs may vary due to several factors, including the type of service provided over the link and the user type. Bandwidth should be allocated to high priority services in a sufficiently short time frame to maintain the quality of service specified by the CPE. The bandwidth request/grant protocol method and apparatus should correct itself when a bandwidth request is lost due to the noise or interference effects present on an air link.